The human polyomavirus BK virus infects a high proportion of the general population and remains latent in the kidney after primary infection. Reactivation can occur when T cell functions are deficient (e.g., in recipients of bone marrow or organ transplants). BK viruria has also been implicated as a cause of hemorrhagic cystitis in bone marrow transplant patients. Rapid detection of active BK virus may help in the medical management of these patients. A 72-hour shell vial culture assay for BK virus was developed to evaluate the incidence and significance of BK virus in the bone marrow transplant patient population. Development of a polymerase chain reaction (PCR) assay for detection of BK virus is under way, and a clinical comparison will be performed to determine whether shell vial culture, PCR, or cytological examination of exfoliated urinary epithelial cells is the most sensitive and rapid method for detecting BK virus.